1. Field of the Invention
The present invention relates to a semiconductor device having a semiconductor layer on a semiconductor substrate and a method of manufacturing the same.
2. Description of the Prior Art
Many of current semiconductor devices are manufactured using monocrystalline semiconductor substrates or epitaxial wafers each of which is obtained by growing only one epitaxial layer on a monocrystalline semiconductor substrate. In manufacturing semiconductor devices in an epitaxial wafer, since electrical active layers having a resistivity different from that of a semiconductor substrate can be formed on the semiconductor substrate, a degree of freedom of manufacturing the semiconductor device is large. Furthermore, a high-purity monocrystalline thin film having low concentrations of oxygen and carbon which cause crystal defects can be advantageously formed to have an arbitrary thickness.
For this reason, the epitaxial wafer is practically used for a high voltage semiconductor device, a bipolar integrated circuit device, a CCD or the like. When an epitaxial wafer is particularly used for a CCD, a high-resistive epitaxial layer of the same conductivity type as that of a low-resistive semiconductor substrate is formed on the semiconductor substrate, thereafter forming a CCD. Therefore, a substrate voltage serving as an electronic shutter voltage or the like can be much reduced as compared with a case wherein a CCD is formed in only a high-resistive semiconductor substrate. In the epitaxial wafer, P is conventionally doped as an impurity in the semiconductor substrate.
When a semiconductor device is formed using an epitaxial wafer obtained by sequentially stacking two epitaxial layers, i.e., a low-resistive epitaxial layer and a high-resistive epitaxial layer, on a semiconductor substrate, the semiconductor device is not theoretically limited by the characteristics of the semiconductor substrate, and nonuniformity of the impurity concentration of the semiconductor substrate is not reflected on the characteristics of the semiconductor device. For this reason, it is understood that the semiconductor wafer has an ideal structure. In the above epitaxial wafer, P is conventionally doped as an impurity in the lower epitaxial layer.
FIG. 1 shows a relationship between the thickness of an upper epitaxial layer and a shutter voltage of a CCD when the CCD is formed in an epitaxial wafer obtained by stacking two epitaxial layers on a semiconductor substrate in the same manner as described above. As is apparent from FIG. 1, unless the thickness of the upper epitaxial layer is 10 .mu.m or less, an effect of reducing the shutter voltage cannot be obtained. The minimum thickness (i.e., 4 .mu.m) of the upper epitaxial layer is required to form the CCD.
On the other hand, FIG. 2 shows a ratio of the impurity concentration of the lower epitaxial layer to the impurity concentration of the upper epitaxial layer when the thickness of the upper epitaxial layer is fixed to 8 .mu.m and the impurity concentration of the upper epitaxial layer is fixed to an optimal value at which the CCD can be operated. That is, as is apparent from FIG. 2, unless the ratio of impurity concentrations is 10 or more, an effect of reducing a shutter voltage cannot be obtained.
However, the diffusion coefficient of P is large. For this reason, when the epitaxial wafer having one epitaxial layer according to the first prior art is annealed at a high temperature, P is diffused from the semiconductor substrate to the epitaxial layer, an epitaxial layer having a stable impurity concentration cannot be formed. In addition, since only one epitaxial layer is formed, the nonuniformity of the impurity concentration of the semiconductor substrate is reflected on the characteristics of the semiconductor device, and nonuniformity of an image occurs in a CCD. Therefore, in the epitaxial wafer of the first prior art, a semiconductor device having uniform characteristics cannot be formed.
When an epitaxial layer is formed on a semiconductor substrate in which As is doped as an impurity, an amount of impurity diffused from the semiconductor substrate to the epitaxial layer can be reduced, thereby obtaining an epitaxial layer having a stable impurity concentration. However, the nonuniformity of the impurity concentration of the semiconductor substrate is still reflected on the characteristics of the semiconductor device. In addition, since the toxicity of As poses a safety problem, the semiconductor substrate doped with As is less popular than a semiconductor substrate doped with P.
Since the epitaxial layer is generally contaminated with heavy metals while the epitaxial layer is grown, a generation lifetime of an electrical active layer is shortened due to contamination near the surface of the epitaxial wafer. That is, a time from generation of carriers to their recombination is 5 msec or less which is shorter than 10 msec of the semiconductor substrate which is grown by a CZ method. For this reason, a semiconductor device having excellent characteristics cannot easily be formed, and an increase in white dot or dark current is recognized in the CCD.
In the epitaxial wafer having the two epitaxial layers according to the second prior art, when the thickness of the upper epitaxial layer is decreased to 10 .mu.m or less as described above, P is diffused in the whole upper epitaxial layer. That is, as indicated by a dotted line in FIG. 4, the resistivity begins to be decreased near the surface of the upper epitaxial layer, and a CCD cannot easily be formed on the upper epitaxial layer. In addition, heavy-metal-based contamination also occurs in the epitaxial wafer of the second prior art.